Although geothermal wells have different pH values from place to place, when a geothermal well spouts a strong-acid geothermal fluid, the development of such a geothermal well is usually stopped. Among acid geothermal fluids of pH 7 or less, “a strong-acid geothermal fluid” herein refers to a fluid of pH 5 or less, which is known to corrode carbon steel materials badly. As a rare example, however, steam is produced by injecting an alkali solution into such a geothermal well to neutralize acid hot water, examples of which are introduced in Non-Patent Literatures 1 to 3.
Such an acid geothermal well typically has a high-temperature strong-acid atmosphere at temperatures of about 200 to 300° C. and having pH of about 2, whereas an alkali solution to be injected is a high-density solution of 20 to 50 mass %, having pH up to 14 in some cases.
A solution injection pipe that is used for the injection of such an alkali solution is called a neutralization pipe as well because it is inserted in a casing pipe for injection of an alkali solution into an acid geothermal well for neutralization, which is typically a single pipe made of Ni-based alloy because it has to have both of acid resistance and alkali resistance.
However, it is known that such a single pipe made of Ni-based alloy does not have sufficient corrosion resistance in a high-temperature strong-acid atmosphere of a geothermal well, and so currently such a single pipe made of Ni-based alloy is used while being replaced in a relatively short period, such as one to two months. Currently therefore the cost required for the replacement of a single pipe is a big problem.
Then, the usage of a single pipe made of a Ti-base material, which can be resistant to a high-temperature strong-acid environment as well, may be considered. Ti, however, is susceptible to corrosion in a high-temperature alkali environment, and so a countermeasure will be needed, such as the formation of a coating made of a high-temperature alkali resistive material on the inner face of the single pipe through which an alkali solution flows.
Patent Literatures 1 and 2, for example, disclose the technique of painting with fluorine resin as a high-temperature alkali resistive material on the inner face of a titanium pipe. Patent Literature 3 discloses a coated pipe prepared by applying a coating of fluorine resin to the inside of a metallic pipe.
The techniques of painting fluorine resin or of applying a coating of fluorine resin to the inside of a single pipe as disclosed in Patent Literatures 1 to 3 have the following problems.
That is, one of the problems resides in that there is no technique established, of applying a coating of fluorine resin of a desired thickness (e.g., 0.2 mm or more) on the inner side of a long steel pipe that is relatively thin of 50 mm or less in diameter and extends up to 2,000 to 3,000 m without pinholes.
Another problem resides in that water in an alkali solution may permeate through the fluorine resin layer and may be accumulated at the contact interface between the fluorine resin layer and the steel pipe, and such a solution accumulated gradually may increase in size and may damage the fluorine resin layer or the fluorine resin layer may swell to block the hollow inside of the single pipe and so inhibit the flow of the alkali solution.
Still another problem resides in that, if the neutralization pipe is partly damaged, the inserted pipe as a whole extending up to 2,000 to 3,000 m has to be removed out to the ground irrespective of the site of the damage, which then has to be replaced with a new neutralization pipe.